This invention relates to the art of oil extraction from a vegetable oil bearing material such as soybean, corn and the like, and more particularly, to a method and assembly for pretreating oil bearing vegetable material, extracting the oil therefrom, and producing a superior quality vegetable oil suitable for physical refining.
The invention is particularly applicable to the processing of oil from soybeans and corn germ, but is also applicable to many other vegetable oil bearing materials such as cottonseed, peanuts, sunflower seed, rape seed, fresh coconut meats or dried coconut meats, palm fruits and palm kernels and the like. The process of the present invention improves the extractability of the vegetable oils from the oil bearing materials while producing an oil product very low in phospholipids and in mineral content such as, specifically, calcium, magnesium and iron. The oil product is thus amenable to physical refining. However, it will be appreciated by those skilled in the art that the invention can be readily adapted for use with other extraction processes as, for example, where similar methods are employed to obtain other types of valuble constituent products.
Soybeans dominate the United States and world oil and vegetable protein markets and, accordingly, conventional vegetable oil processing techniques are predominantly directed to soy oil processing. Soy oil and soy protein offer maximum benefit to the consumer at a lower cost than can be obtained from any of the other major oilseeds.
A wealth of information exists describing the conventional methods and equipment used in vegetable oil processing. The commercially viable and successful techniques for soy oil processing entail a number of processing steps to extract the oil. Several techniques exist for the extraction of oil including solvent extraction, mechanical pressing, or a combination thereof, although the dominant technique in commercial use today is solvent extraction.
The crude oil extracted through these various known techniques is a dark colored, turbid liquid with an unacceptable odor and flavor. The liquid needs substantial further treatment to convert it to a bland, stable and nutritious product that is useful in the manufacture of shortening, margarine and salad and cooking oils. (Crude oils from other oilseeds are generally equally unacceptable as a food product and equally need to be further treated.) This further treatment consists of a number of steps which collectively may be called the refining process and which typically include such steps as degumming, neutralizing (alkali refining), bleaching and deodorization. Refining is necessary to remove phospholipids, free fatty acids, color bodies and other constituents which either affects efficient execution of any subsequent processing steps and/or affect the quality and the stability of the oil as a food product.
The crude oils produced by conventional solvent extraction and mechanical pressing methods from soybeans typically contain high levels of phosphorus compounds commonly called phospholipids, phosphatides, phosphoglycerides or gums in the range of 500 to 800 PPM (parts per million measured as phosphorus) and small but significant quantities of calcium, magnesium and iron. As much as 30% of the above phospholipids may be complexed with calcium and magnesium. These are commonly called non-hydratable phospholipids. In addition, it is generally known that prior methods of pretreatment and oil extraction of soybeans are, in fact, conducive to increasing the quantity of non-hydratable phospholipids present in the crude oils produced. The non-hydratable phospholipids generally require a separate degumming step in the refining process for their removal as will be discussed below. It is also well-known by those knowledgeable in the art of refining crude oils that the varying quantities of phospholipids in the crude oils may be attributed to variations in the extraction processes themselves and to the varying compositions of soybeans incurred during the growing, the harvest and the storage of the beans.
Since it is well-known that the presence of phospholipids and certain trace metals are undesirable to the quality of the final food grade vegetable oil, it is advantageous to reduce the level of these compounds as much as possible during the oil extraction processing.
The scope of processing steps referred to above, i.e. degumming, neutralization (alkali refining), bleaching and deodorization are often collectively called "refining." In a narrower use of the word "refining", it is often defined as the technique for neutralizing the free fatty acids in the oil. As this is done with alkali, the technique is also referred to as alkali refining, or because of the use of chemicals, as chemical refining. It should be kept in mind that each processing step generally affects more than one property of the crude oil. While neutralization primarily reduces free fatty acid levels, gums are also removed, the color may become lighter and some odor compounds may be removed. It is this propensity of a particular processing step to affect a variety of oil properties which makes it difficult to predict the complete cause and effect of the processing step and thus is accountable for the inconsistent results obtained from prior processing methods.
The typical known vegetable oil refining process involves several steps including a "degumming" step which essentially comprises adding water to the crude oil and heating and agitating the mixture for a period of time (approximately 10-30 minutes) and at temperatures of typically 50-70 degrees Centigrade. This mixture of hot oil and water is subjected to centrifugation wherein the water and oil are separated. In the process the hydrated phospholipids are separated with the water. The resulting partially "degummed" oil typically still contains a quantity of phospholipids, including all the non-hydratable phospholipids. This quantity may typically contain the equivalent to 10 to 120 PPM of phosphorus, however, this quantity varies depending upon the precise degumming techniques and conditions used.
The partially "degummed" oil produced in accordance with the above process may be further "degummed" to remove the non-hydratable phospholipids by the addition of certain chemicals (such as phosphoric acid) and water and by again heating and agitating the mixture followed by centrifuging. THel "degummed" oil produced from this step will typically contain a quantity of phospholipids equivalent to 5-20 PPM of phosphorus.
The degummed crude oil from this second refining step is further subjected to several additional refining steps to remove other unwanted constituents such as the free fatty acids, the color bodies and other materials that contribute unwanted flavor, color and odor and which cause flavor reversion. These steps are more commonly identified as saponification of free fatty acids, washing of the oil to remove the soaps, neutralization and further washing to remove excess chemicals and soaps and further reduce the quantities of phospholipids, bleaching to remove color bodies and some additional quantities of phospholipids and, finally, deodorization. Oil produced from all of these extracting and refining steps is usedful as a food product but still contains phospholipids equivalent to 1-10 PPM of phosphorus.
It should be particularly emphasized and noted in considering the subject invention that all of these prior known processing steps, and in particular the degumming steps, are applied to a crude oil product already extracted from the oil bearing vegetable material. The steps are not applied to the material itself but to the crude oil extracted from the material.
The capital cost associated with equipment to practice these refining steps is very high. Chemical refining involves many steps which are cumbersome, is capital intensive in that it requires substantial equipment which is hard to maintain such as centrifuges and filter presses, and is inherently characterized by oil losses as each of the refining steps produces a residue which carries with it a certain quantity of usable oil thus decreasing the yield of the salable food product oil.
Because of the high cost of equipment, the high operating expense and the losses of valuable product oil, there has been an emphasis and desire in recent years to practice a technique commonly called physical refining. In this technique a crude oil which has been subjected to several pretreatment processing steps is brought to an elevated temperature (250 degrees Centigrade or more) in a vessel or column operated under vacuum. Steam is sparged into the oil during treatment. Temperature and retention time conditions are selected such that the free fatty acids and other impurities and odiferous compounds are volatilized and distilled off. The treated oil is then typically cooled and given a post bleach to further lighten the color of the oil.
The capital cost and operating costs of a physical refining step is for many crude oils considerably less than that of chemical refining. Oil losses are also substantially less because only unwanted impurities are distilled off. Generally, very little post physical refining treatment is necessary to produce the finished shelf product. Hence, physical refining is very desirable to an oil processor.
However, a number of crude oils, including crude oils from soybean and corn germ extraction, require substantial pretreatment steps before the physical refining step can be applied. Most of these pretreatment steps are associated with the removal of hydratable and non-hydratable phospholipids from the crude oil.
Physical refining does not remove significant quantities of gums or phosphorus, nor does physical refining remove the heavy metals (such as iron). The presence of gums in excess of 6-20 PPM of phosphorus are subject to breakdown during physical refining due to the high temperatures employed and this causes unwanted flavor and color characteristics and causes acceleration of flavor reversion or rancidity (in the case of soy bean oil), as well as a reduction of oil stability (or shelf life) in other vegetable oils. The lower limits of the presence of phopholipids are not quite clear, but it is well known that there is a direct relationship of flavor reversion and loss of shelf life due to the presence of excessive quantities of phospholipids and of heavy metals such as iron in all vegetable oils. Therefore, the feed to a physical refining step should not contain a quantity of phospholipids measured in excess of 3-10 PPM measured as phosphorus. Those knowledgeable in the art may agree that high levels of phospholipids in the feed to the physical refining step cause deep set color changes in the oil which are hard to bleach out. The need for reduction of the phospholipid level in corn and soybean crude oils requires many of the prior art chemical refining steps described earlier and thus much or all of the economic incentive for physical refining is lost.
The application of physical refining is therefore limited to those vegetable oils that are naturally of such a quality as to have low limits of phosphorus (particularly the non-hydratable phospholipid form), have a low iron content and, in addition, contain a level of free fatty acids dictated by economic justification to permit the full application of phyisical refining or some modification thereof.
A major reason for not applying the physical refining step to soybean and corn oil crudes is that these crudes are high in phospholipids and in the case of corn oil contain much foreign solid matter such as finely divided starch particles. High levels of phospholipids in the crude affect the quality of the oil and generally limits have been set on the maximum phospholipid levels for physical refining of a crude oil. These requirements set by the refiners of crude oil range from less than 5 PPM (measured as P) to less than 20 PPM.
As noted above, the reduction of the quantities of phospholipids in soybean oil and corn oil crudes is not an easy task because part of the phospholipids are in a form generally referred to as non-hydratable phospholipids or may be converted to this form under the influence of certain constituents of the oilseeds or the oil. The greater part of the phospholipids generally referred to as hydratable phospholipids may be removed readily by contacting the crude with water, salt solutions, acidic or caustic solutions and the like and then removing the agglomerations of hydratable phospholipids by means of centrifuging. The removal of the non-hydratable phospholipids is more difficult. The non-hydratable phospholipids are complexes of calcium and magnesium with phospholipids and the known removal techniques depend upon chemical treatments to cleave the bond between the calcium and magnesium groups and the phospholipids, rendering the non-hydratable phospholipids into hydratable phospholipids and preventing reattachment of the calcium and magnesium group to the hydratable phospholipids.
The present invention contemplates a new and improved method and assembly which allows for the more efficient processing of a better quality oil product and meal product from a vegatable oil material.
As a result of the process according to the present invention the phospholipids substantially remain with the extracted solids. The extracted crude oil is very low in phospholipids and may be physically refined without any further pre-treatments.